1. Field of the Invention
The present invention relates to a copper foil for an inner layer board of a very thin, fine pitched pattern multi-layer board and a process for producing the same. More particularly, it relates to a printed circuit inner-layer copper foil having low surface roughness with fine nodules formed on both surfaces of the foil thereby to make it possible to obtain fine pitched pattern circuits by etching and have high adhesion to a substrate and also to a process for producing the same.
2. Prior Art
Conventional multi-layer boards have been produced by bonding ordinary inner-layer copper foils (where mat (rough) surface roughness Rz is around 7.0 .mu.m and shiny (smooth) surface roughness is around 0.7 .mu.m) having nodules formed on their mat surface to substrates facing to the mat surface, preparing circuits by etching, subjecting the circuits to black oxide treatment, adhering substrates respectively to the outsides of the circuits under heat and pressure, adhering copper foils respectively to the outsides of these substrates under heat and pressure, and then etching the outer copper foils to form circuits thereby to prepare a four-layer printed circuit board. To prepare five- or more-layer printed circuit (multilayer) printed circuit boards, the above steps may be repeated in turn. Alternatively, a copper foil with nodules formed on both side thereof (where mat surface roughness Rz is around 8.5 .mu.m and shiny surface roughness Rz is around 1.6 .mu.m) is commercially available as a both surfaces-treated copper foil.
However, in the above-mentioned black oxide treatment, a haloing phenomenon tends to take place since the oxide film is solved in a hydrochloric acid solution. Thus, the insulating characteristics and the reliability of layer-to-layer interconnection will tend to be deteriorated. To overcome such defects, the black oxide treatment may be followed by a chemical reduction treatment. The black oxide treatment should be performed using a concentrated alkaline solution and, in addition, the reduction treatment should be performed whereby the production cost will increase.
On the other hand, since the conventional both surfaces-treated copper foil has copper nodules formed on both the surfaces thereof, the above-mentioned problem is not raised. However, since this treated copper foil is prepared from a conventional copper foil, its mat surface roughness is high. Thus, this treated copper foil is difficult for use in the preparation of fine pitched pattern circuits therein which are now in rapid progress. When the surface roughness of the copper foil at the interface with the substrate is high, the linearity of the foot-line of conductor will decrease after circuits are obtained by etching. Thus, if the conductor spacing is small in this case, a problem such as migration will be occured.
In addition, multi-layer construction and thin board construction have now been in progress. To reduce the thickness of each layer consisting of a copper foil and a substrate therefor, tile thickness of the substrate is also decreased. Further, the decrease of interlayer insulation due to the roughness of the mat surface of the copper foil is strongly sought to be prevented as far as possible.
Since the shiny surface roughness Rz of a conventional copper foil is as low as 0.5 to 1 .mu.m, the strength of adhesion of the copper foil to the substrate will be unable to be improved even if the nodule forming treatment is performed on said surface. When the size of the nodules is increased in an attempt to enhance the adhesion strength, the nodules themselves on the copper foil will raise a problem such as dropping off from the copper foil. In addition, there are apt to be raised secondary problems that the nodules will be left in a substrate after a circuit is prepared by etching or they will be crushed when pressed.